There is an increasing need for high strength polymeric materials to replace metals in many applications. The polymeric materials have the advantage of lower weight and are often less expensive and more durable than metals. Usually, however, the polymeric materials are much lower in strength than the metal materials they replace. To improve the strength of these materials, polymeric materials are reinforced in some manner.
Thus, polymeric composite materials have been developed to meet these strength requirements. These composites are characterized by having a continuous polymer matrix in which is embedded a reinforcement material. This reinforcement material, in many cases, takes on the form of a relatively rigid, high aspect ratio material such as glass fibers.
To get the reinforcement material into the composite, the reinforcement material is usually placed into the mold in a first step, followed by closing the mold and then introducing a fluid molding resin. The resin fills the mold, including the interstices of the fibers, and hardens (by curing or cooling) to form the desired composite. Alternatively, temperature and pressure may be applied to the mold to cure the resin to form the desired composite.
It is highly desirable for the reinforcement material to be uniformly distributed throughout the composite article, thereby preventing weak spots within the composite material where reinforcement is lacking. To accomplish this, it is conventional for the reinforcement to be formed into a mat prior to introduction to the mold and then be introduced to the mold to be impregnated with resin to form the final composite article. The mat, or preform, is generally prepared by forming the reinforcing fibers into a shape matching the inside of the mold and applying a binder to the fibers. In some cases, a thermoset resin in pre-applied and then cured after the fibers are shaped into a mat. More typically, a thermoplastic binder is used, so that in subsequent operation the binder can be heated and softened and the mat subsequently shaped. This binder “glues” the individual fibers so that the resulting mat retains its shape when it is transferred to the mold for further processing.
The binders that are used primarily consist of three types. Predominately, binders having solvent-borne polymers, i.e. liquids, such as epoxy or polyester resings. The solvent-borne binders are applied in a spray or dip-coating application followed by a subsequent heating step to volatize the solvent and possibly cure the binders. These binders are undesirable from both an environmental perspective and because the process is energy intensive.
A second method utilizes heated thermoplastic materials which can be melted and sprayed as a binder. Use of these materials make a subsequent heating step unnecessary. However, in this method, “lofting”, or inadequate compaction of the preform, typically occures, because the thermoplastics exhibit a lack of uniformity in their cooling patterns and extensive migration along fiber surfaces. This can lead to density gradients or lower density preforms than desired.
A third class of binders that may be used is powdered binders. Powder binders can be mixed with the fibers and then the mass formed into a preform shape, which is then heated to cure the binder in situ. The binders can also be sprayed to contact the fibers and then heated to melt the powder onto the fibers to form a preform. An advantage to this process is that the powder is effective in going to the junction, or crossover points, of the network of fibrous material. Thus, the powder adequately anchors the fibers together in the preform, but does not interfere with the subsequently applied binder resin along the majority of the fiber used to form the composite part.
Currently available powdered binders that are used in fiber preforms are expensive to manufacture. Also, certain powdered binders are not b-stageable thermosetting powders. A b-stageable thermosetting powdered binder, for the purposes of this invention, is defined as a powder binder which can be heated to a tackifying temperature without crosslinking such that it will stick to and adhere together the glass fiber stands in the system to form the fiber preform. It would thus highly desirable to provide a b-stageable powdered binder system that limits the cost.